Leucism

Leucism (/ˈluːsɪzəm, -kɪz-/)[2][3][4] is a wide variety of conditions that result in the partial loss of pigmentation in an animal—causing white, pale, or patchy coloration of the skin, hair, feathers, scales, or cuticles, but not the eyes.[4] It is occasionally spelled leukism. Some genetic conditions that result in a "leucistic" appearance include piebaldism, Waardenburg syndrome, vitiligo, Chédiak–Higashi syndrome, flavism, isabellinism, xanthochromism, axanthism, amelanism, and melanophilin mutations. Pale patches of skin, feathers, or fur (often referred to as "depigmentation") can also result from injury.[5]

Leucistic white lions owe their colouring to a recessive allele. Note the eyes and lips remain the normal colour. Studies have shown that the reduced pigment comes from a mutation in the gene for tyrosinase, the same as causes Type I oculocutaneous albinism in humans.[1]
All-white dominant white horse with pink skin, brown eyes, and white hooves.
This white horse owes its coloring to a dominant allele (dominant white).
A leucistic rock dove. Both the eyes and legs are still of the normal colour.

Details

Leucism is often used to describe the phenotype that results from defects in pigment cell differentiation and/or migration from the neural crest to skin, hair, or feathers during development. This results in either the entire surface (if all pigment cells fail to develop) or patches of body surface (if only a subset are defective) having a lack of cells that can make pigment.

Since all pigment cell-types differentiate from the same multipotent precursor cell-type, leucism can cause the reduction in all types of pigment. This is in contrast to albinism, for which leucism is often mistaken. Albinism results in the reduction of melanin production only, though the melanocyte (or melanophore) is still present. Thus in species that have other pigment cell-types, for example xanthophores, albinos are not entirely white, but instead display a pale yellow color.

More common than a complete absence of pigment cells is localized or incomplete hypopigmentation, resulting in irregular patches of white on an animal that otherwise has normal coloring and patterning. This partial leucism is known as a "pied" or "piebald" effect; and the ratio of white to normal-colured skin can vary considerably not only between generations, but between different offspring from the same parents, and even between members of the same litter. This is notable in horses, cows, cats, dogs, the urban crow[7] and the ball python[8] but is also found in many other species.

Due to the lack of melanin production in both the retinal pigmented epithelium (RPE) and iris, those affected by albinism sometimes have pink pupil due to the underlying blood vessels showing through. However, this is not always the case and many albino animals do not have pink pupils.[9] The common belief that all albinos have pink pupils results in many albinos being incorrectly labeled as 'leucistic'. The neural crest disorders that cause leucism do not result in pink pupils and therefore most leucistic animals have normally colored eyes. This is because the melanocytes of the RPE do not derive from the neural crest. Instead, an out-pouching of the neural tube generates the optic cup that, in turn, forms the retina. As these cells are from an independent developmental origin, they are typically unaffected by the genetic cause of leucism.

Genetics

Genes that, when mutated, can cause leucism include c-kit,[10] mitf[11] and EDNRB.[12]

Etymology

The terms leucistic and leucism are derived from the stem leuc- + -ism, from Latin leuco- in turn derived from Greek leukos meaning white.

See also

References
  1. Cho, Yun Sung; Hu, Li; Hou, Haolong; Lee, Hang; Xu, Jiaohui; Kwon, Soowhan; Oh, Sukhun; Kim, Hak-Min; Jho, Sungwoong; Kim, Sangsoo; Shin, Young-Ah (2013-09-17). "The tiger genome and comparative analysis with lion and snow leopard genomes". Nature Communications. 4 (1): 2433. Bibcode:2013NatCo...4.2433C. doi:10.1038/ncomms3433. ISSN 2041-1723. PMC 3778509. PMID 24045858.
  2. "leucistic". Oxford Dictionaries. Oxford University Press. Archived from the original on August 10, 2012. Retrieved January 9, 2016.
  3. Merriam-Webster, Merriam-Webster's Unabridged Dictionary, Merriam-Webster.
  4. "leucism". The American Heritage Dictionary of the English Language (5th ed.). HarperCollins.
  5. "Depigmentation of Skin: Symptoms, Signs, Causes & Treatment". MedicineNet. Retrieved 2020-06-23.
  6. chemport.cas.org https://chemport.cas.org/cgi-bin/sdcgi?APP=ftslink&action=reflink&origin=npg&version=1.0&coi=1:CAS:528:DC%252BC3sXotlGksbg%253D&md5=1ee737efa06deefbbb993e3412a921a0. Retrieved 2020-06-23. {{cite web}}: Missing or empty |title= (help)
  7. White crows at Cornell University.
  8. Piebald ball pythons at Constrictors.com (archived 9 October 2006, from the original, accessed 18–22 July 2006).
  9. "Oculocutaneous Albinism". NORD (National Organization for Rare Disorders). Retrieved 2020-06-23.
  10. Coat color, dominant white Archived 2009-01-30 at the Wayback Machine at Online Mendelian Inheritance in Animals.
  11. An L1 element intronic insertion in the black-eyed white (Mitf[mi-bw]) gene: the loss of a single Mitf isoform responsible for the pigmentary defect and inner ear deafness at Human Molecular Genetics.
  12. Waardenburg syndrome at Atlas of Genetics and Cytogenetics in Oncology and Haematology (archived 26 December 2005, from the original, accessed 18–22 July 2006).
  • Media related to Leucism at Wikimedia Commons
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